This invention relates to a method of adjusting the coefficients of a filter means, in which method an input signal is supplied to the filter means, a resulting signal derived from the filter means is compared with a reference signal, and the coefficients are adjusted to reduce the difference between the compared signals. The invention also relates to an arrangement comprising a filter means provided with coefficient adjusting means for, when activated, supplying an input signal to the filter means, comparing a resulting signal derived from the filter means with a reference signal, and adjusting the coefficients to reduce the difference between the compared signals.
Filters are often employed, for example, as so-called "equalizers" for compensating for distortion to a transmitted signal caused by the imperfect nature of a transmission channel. If the channel characteristics vary with time then, in order that satisfactory equalization continues to be achieved, it is necessary that the filter transfer characteristic be updated either effectively continuously or periodically to take into account the changed channel characteristics. In the latter case it is often arranged that a standard signal, i.e. one which is known at the receiver, is transmitted periodically. Each time this standard signal is received, albeit in possibly distorted form, it can be passed through the receiver equalizing filter and the result compared with what it ideally should be. Any differences revealed by the comparison can be used to adjust the filter coefficients in such a way as to reduce these differences, i.e. in such a way as to create a transfer characteristic for the filter which is such as to convert the distorted signal received substantially into the signal originally transmitted. These adjusted coefficients are then maintained until the standard signal is received once again, at which point another adjustment to the coefficients is performed. Many algorithms have been proposed for achieving the required coefficient adjustments. In general these algorithms cause the coefficients to approach the required values as successive portions of the standard signal are received and processed, the rate at which this approach occurs being a function of the particular algorithm employed. Those algorithms which give a comparatively fast rate of approach tend to require very large amounts of computation whereas those algorithms which are more reasonable in respect of their demands on computing power tend to give a comparatively slow rate of approach. In consequence, if the amount of computation is to be kept to within reasonable bounds an algorithm which gives a comparatively slow rate of approach has to be used, with the result that the "standard signal" has to be, each time it occurs, of a comparatively long duration to enable the algorithm to achieve the accuracy of which it is inherently capable. However, employing a "standard signal" of a comparatively long duration reduces the efficiency of transmission through the transmission channel, because the standard signal displaces useful information which could otherwise be transferred at the relevant time.